Test and measurement instruments such as spectrum analyzers, oscilloscopes, and logic analyzers have “trigger event detectors” that allow them to capture and analyze portions of a signal-under-test before, at, and after specific events occur referred to as “trigger events.” Trigger events can be detected from the signal-under-test itself or from a secondary signal. In order to insure a high probability of detecting a trigger event, a trigger event detector must operate continuously, regardless of whether a trigger event is present or not. FIG. 1 illustrates a spectrum analyzer 100 having such a trigger event detector 105. In operation, a signal-under-test is conditioned by a signal conditioner 110 (e.g., by down-converting the signal-under-test from RF to IF) and then digitized by an analog-to-digital converter (ADC) 115 to produce a continuous stream of digitized samples. The digitized samples are written into a circular buffer 120 and also input to the trigger event detector 105. The trigger event detector 105 processes the digital samples and then compares the processed samples to a user-specified trigger threshold. When the processed digital samples exceed the trigger threshold, the trigger event generator 105 generates a trigger signal that causes an acquisition memory 125 to capture the digitized samples that are held in the circular buffer 120. The captured samples are then analyzed by a display processor 130 (e.g., by transforming them into the frequency domain using a frequency transform such as a fast Fourier transform (FFT), a chirp-Z transform, or the like) and displayed on a display device 135. The trigger event detector 105 may be any one of various kinds of trigger event detectors that are used to detect various kinds of trigger events. For example, the trigger event detector 105 may be a “power level trigger” that detects when the instantaneous power of the signal-under-test exceeds a user-specified power threshold. In that case, the trigger event detector 105 processes the digitized samples by converting them into a measure of the instantaneous power of the signal-under-test, and the user-specified trigger threshold is a power threshold. When the instantaneous power of the signal-under-test exceeds the power threshold, the trigger event generator 105 generates the trigger signal.
In order to detect more complex trigger events, a trigger event detector may need to be more selective than the acquisition path of the instrument (i.e., the signal conditioner, ADC, circular buffer, acquisition memory, and so on). For example, as shown in FIG. 2, a spectrum analyzer 200 may include a band-pass filter (BPF) 240 that filters the signal-under-test before it is applied to the trigger event detector 205 so that the trigger event detector 205 only detects trigger events within a narrow range of frequencies (as described in U.S. Pat. No. 5,493,209 titled “Tunable trigger acquisition system and method for making in-service time-domain signal measurements”). In this manner, the bandwidth of the signal received by the trigger event detector 205 is much narrower than the bandwidth of the signal-under-test displayed on the display device 235.
The deficiency in these conventional instruments is that it is difficult for a user to ascertain how to set up and arm the trigger event detector. That is, the user cannot see the signal-under-test on the display device until a trigger event occurs. Thus, if the trigger event detector is set up improperly or if the signal-under-test changes after the user arms the trigger event detector, then the user has no way of ascertaining why the instrument is not triggering. This deficiency is particularly problematic in the case where the trigger event detector is more selective than the acquisition path of the instrument because, even if the user can somehow get the instrument to display the signal-under-test (e.g., by forcing a trigger), the signal-under-test shown on the display device does not necessarily correspond to the signal received by the trigger event detector.